Stable positively charged alumina coated silica sols and their preparation by postneutralization

ABSTRACT

An improved process for preparing a stable positively-charged silica sol is obtained by mixing a negatively charged silica sol with basic aluminum chloride, heating the mixture between 45* to 90*C. and then adding to the mixture an alkali metal base, alkaline earth metal base, ammonia or water soluble organic base in the amount of 0.032 to 0.223 equivalents of base per equivalent of aluminum.

United States Patent Moore, Jr. 5] March 6, 1973 STABLE POSITIVELYCHARGED [56] References Cited ALUMINA COATED SILICA SOLS AND THEIRPREPARATION BY POST- UN'TED STATES PATENTS NEUTRALIZATION 3,252,9l75/1966 Mindick et al. ..252/3l3 S 2,892,797 6/1959 Alexander et a]...252/3l3 S Inventor: Earl Moore, J g m 2,702,787 2/1955 Freeland..252/309 5 1361- 2,572,578 10/1951 Trail .....252/313 S 07, ..252 l[73] Assrgnee: E. l. du Pont de Nemours and Com- 3 O 878 11/1961Alexander et B 3 S pany wllmmgton Primary Examiner-Richard D. Lovering[22] Filed: J 29, 1971 Attorney-James L. Jersild Related US. ApplicationData An improved process for preparing a stable positivelycharged silicasol is obtained by mixing a negatively [63] Commuauon-m'pan 9 Junecharged silica sol with basic aluminum chloride, heatl969, abandoned,which IS a continuation-impart of o o Ser. 745,714, July 18 1968abandoned mg the mixture between 45 to 90 C. and then adding to themixture an alkali metal base, alkaline earth metal base, ammonia orwater soluble organic base in the amount of 0.032 to 0.223 equivalentsof base Per 58 Field of Search .,252/313 s, 313 R i r alummum' 9 Claims,No Drawings STABLE POSITIVELY CHARGED ALUMINA COATED SILICA SOLS ANDTHEIR PREPARATION BY POST-NEUTRALIZATION CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a continuation-in-part of U.S. Pat.application Ser. No. 831,748, filed June 9, 1969, now abandoned, whichis a continuation-impart of U.S. Pat. application Ser. No. 745,714,filed July 18, 1968, now abandoned.

BACKGROUND OF THE INVENTION Positively charged coated silica particlesare known to the art as first disclosed in U.S. Pat. No. 3,007,878 toAlexander and Bolt. These novel'positively charged coated silicaparticles have a variety of unique properties. They are distinctlydifferent than the previously known negatively charged silica andmodified silica sols. However, stability has presented problems whichlimited their commercial manufacture.

A method has been proposed to increase the stability of these positivelycharged particles as represented by U.S. Pat. No. 3,252,917 to Mindickand Thompson. While this patent discloses a new technique for preparingthese positively charged particles it is costly and time consuming. Theprocess of Mindick et al. requires a minimum of five steps not includingregeneration of the two types of ion exchange resins employed in theprocess. Further, the process step of mixing deionized silica sol withbasic aluminum chloride where the sols colloidal particles are about 18millimicrons or less in diameter results in mixtures of high viscosity.Where the product of this step has a solids content of about 22 percentor higher the viscosity reaches levels of about 10,000 centipoises. Suchviscous process intermediates are extremely difficult to handle insucceeding process steps. However, Mindick et al. requires as anabsolute necessity for stable products the use of deionized silica solsand the final step of removal of anions from the sol. Thus, in order toachieve stable sols with pI-Is in the range of 4.5 to 6 Mindick et al.teaches a difficult procedure and precludes the use of a more efficientprocess to achieve comparable or superior results.

This invention permits the efficient production of stable positivelycharged sols in three steps without encountering any processintermediates having a viscosity higher than about 50 centipoises evenfor the most concentrated products. Further, the sols of this inventionare stable at high conductivities, 30-40,000 micromhos/cm. at 70 F.,unlike the sols of Mindick et al., and thus may be used in contact withmaterials with leachable salts.

. SUMMARY OF THE INVENTION A sol is prepared by mixing a negativelycharged silica sol with basic aluminum chloride as taught in U.S. Pat.No. 3,007,878 to Alexander and Bolt. This mixture is then aged bystanding at room temperature for several weeks or is heated at atemperature up to the boiling point of water with a resulting pHdecrease. (Higher temperatures would be acceptable if no water is lost.)The ageing is conducted until the pH remains essentially constant whichgenerally occurs at a pH of 4.2 or below. The mixture is then stabilizedby adding from 0.032 to 0.223 equivalents of an alkali metal base, analkaline earth metal base, ammonia, water soluble organic bases ormixtures of these bases per equivalent of aluminum. This addition raisesthe pH to about 4.0 to 6.5. At high dilutions the mixture can bestabilized by addition of 0.25 equivalents of base per equivalent ofaluminum. The pH can be as high as 7.0.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The silica sols used as aninitial ingredient are commercially available such as those sold underthe registered trademark Ludox. Typical processes for preparing thistype of silica sols are disclosed in Bechtold and Snyder, U.S. Pat. No.2,574,902; Rule, U.S. Pat; No. 2,577,485 and White, U.S. Pat. No.2,285,477.

The silica sols are mixed with a basic aluminum chloride. Suitable typesof basic aluminum chloride are AI(OH) C1 to Al (OI-I) Cl. The preferredchloride is Al (OI-I) -,Cl. The basic aluminum chloride is usually inthe form of an aqueous solution when mixed with the silica sol. Theprocedure of preparing these mixtures is disclosed in Alexander et al.,U.S. Pat. No. 3,007,878. Example 1 of this patent is exemplary ofpreparing the mixtures utilized in the present invention.

In preparing the mixture of positively charged alumina coated silicaparticles used in this invention, the aluminum to surface SiO mol ratiois usually about 1:2 to 2:1 and preferably 121.25 to 1.25:1 with themost preferred ratio being 1:1. The silica particle size is about 2millimicrons to 150 millimicrons and preferably 7 to 30 millimicronswith the most preferred size being 10-15 millimicrons.

The mixture which is obtained by combining the reactants described aboveis believed to be comprised of silica particles partially coated withpositively charged aluminum and free, unattached aluminum salts. Thismixture is then aged. Ageing allows more of the free aluminum salt to beadsorbed by the silica. A concommitant reduction in the pH value of themixture occurs until an equilibrium state is attained. Ageing can beaccomplished by letting the mixture stand for several days at roomtemperature or by heating the mixture at a temperature up to the boilingpoint of water. The ageing is continued until the decreasing pH of themixture remains essentially constant. The. time required to achieve theconstant pH level decreases proportionately with an increase in ageingtemperature. Thus, the essentially constant pH is obtained at 25 C. inabout hours, at 45 C. in about 6 hours, at 60 C. in about 2 hours, at 80C. in about 30 minutes and at C. in about 15 minutes. Preferably theageing temperature is between about 45 to 90 C.

After the pH of the mixture is reduced by heating, an appreciablepercentage of the charged aluminum in the mixture still exists as freematerial, unattached to the colloidal silica particles. It is necessaryto reduce this percentage of free aluminum salts by forcing them ontothe surface of the silica in order to attain a truly stable positivelycharged colloid dispersion or positive sol.

Basic material is added in an amount proportional to the amount ofaluminum in the mixture. The amount of base added can be as low as 0.032equivalent per equivalent of aluminum for a positive effect on stabilityto be seen, or base can be as high as 0.223 equivalent per equivalent ofAl, or even higher with more dilute sols. Preferably, the amount ofbasic material added is 0.09 to 0.17 equivalent per equivalent of Al.The final pH of the positively charged silica sol will usually drop uponstanding; this has no effect on stability, however; it is only necessaryto add enough basic material as discussed.

The most common basic materials for accomplishing this vital, final stepin this inventive process are alkali metal bases such as sodiumhydroxide, potassium hydroxide and lithium hydroxide; alkaline earthmetal bases such as magnesium hydroxide and calcium hydroxide or theoxides of these metals which, of course, become hydroxides in water;ammonia; water soluble organic bases such as methylamine andethanolarnine. The preferred basic material is magnesium hydroxide (oroxide). Basic materials can be added as aqueous solutions or dispersionsor as solid, finely divided reagent; ammonia can be added as a gas, ascan volatile organic bases.

The improved process of this invention yields positively-charged silicasols which exhibit excellent stability. This property is extremelycritical to the effectiveness of the sol as a commercial product.Positive sols of high solids content, with solids expressed as SiO A1 ashigh as 45 percent, and with outstanding stability can be prepareddirectly without a final concentrating step. The presence of saltsformed during this inventive process in no way detracts from thepositive sol properties or stability. Positive sols prepared by thisinventive process have been stored for as long as four years without anydetrimental effects of gelling or precipitation of the sol components.

AMP E One hundred thirty two pounds ofLudox- HS colloidal silicacontaining 40% SiO by weight, the silica particles having an averageparticle size of 12 to millimicrons and a specific surface area of about215 m /g. SiO is adjusted to pH 7.50 with 821 g. ofa 1:1 mixture ofconcentrated hydrochloric acid and water.

The silica sol then is mixed with 30.5 lbs. of 50 percent chlorhydrol(Al (Ol-l Cl) and 22.2 lbs. water by introducing it at a rate oflb./min. into a centrifugal pump circulating the basic aluminum chloridesolution. A clear, intermediate product of'pH 4.25 and 15.0 cpsviscosity is obtained. A sample of this intermediate product, which isdescribed in US. Pat. No. 3,007,878, gelled within 2 weeks when storedat 140 F.

This product is heated to 70 C. over a, period of 45 min. and at 70 C.for 1 hr. To the cooled, circulated sol of pH 3.80 finally is addeddilute ammonium hydroxide sol on (2.3 NH th ough a in et tube n the pumpto give 0.14 equivalents N151; per equivalent alummum.

The clear, exceptionally stable positive sol Contains 25.96% SiO,, 4.11%A1 0,, 1.35% Cl, and 0.19% N11 lt is stable for 4. months. at 140 F. Themol ratio. of Al to surface silica is 1:1. The sol has a viscosity. of8.0 cps and a specific conductivity of- 29,000, micrornhos/cm, at 75 F.

EXAMPLE 2 Two hundred sixty four pounds of the silica sol described inExample 1 is mixed with 62.8 lbs. 50 percent chlorhydrol and 61.7 lbs.water using the centrifugal pump system described in Example 1. Theclear, fluid intermediate product is heated to C. in onehalf hr. and at60 C. for 2 hrs., cooled to 20 C., and stirred with a lightening mixeras well as circulated with the pump as 600 g. magnesium hydroxidedispersed in 1,800 g. water is introduced in 5 min. This is equal to0.139 equivalents Mg(OH)2 P r equivalent Al. Agitation and circulationis continued for 2 hours.

The clear, very stable product contains 26.4% SiO 4.2% A1 0 1.0% Cl, and0.23% MgO. The mol ratio of Al to surface S10 is 1:1. The product has aviscosity of 15 cp and a specific conductivity of 30,500 micromhos/cm,at F.

EXAMPLE 3 Six hundred grams of Ludox TM colloidal silica containing 50%SiO by weight, the silica particles having an average particle size of21 to 25 millimicrons and a specific surface area of about 130 m /g, isdiluted to 37. 6% SiO with 197 g. water and the S01 is added to 210 g.25 percent chlorhydrol in a belnder. The product has a pH of 4.70 andviscosity of 7.0 cp.

The product is then warmed at C. for 30 min., cooled, and a diluteammonium hydroxide solution is added to the product in a blender. 0.138equivalents of NH per equivalent of A1 is added.

The clear sol is stable for 5 months at 140 F. and has a composition of29.8% SiO 2.5% A1 0,, 0.9% Cl, and 0.115% N11 The mol ratio of Al tosurface S10, is 1:1.

' Viscosity of the sol is 7.5 cp and the specific conductivity is 15,200micromhos/cm.

EXAMPLE 4 Two thousand grams of the silica sol described in Example 3 ismixed with 355 g. chlorhydrol and 359 g. water in a large blender. Theclear intermediate product has a pH of 4.25 and a viscosity of 40 cp.

The product is warmed for about 5 hours at 45 C., cooled to. 25 C. and aslurry of magnesium hydroxide is added to give 0.156 equivalent Mg(OH)per equivalent Al.

The clear, very stable sol has a composition of 34.6% $102, 3.24% A1 0101% C1, and 0.18% MgO. The mol ratio of Al to surface SiO is 1:1.Viscosity of the sol is 23.4 cp and specific conductivity is 18,200micromhos/crn. at 75 F.

EXAMPLE 5 Seven hundred fifty grams of Ludox" LS colloidal silicacontaining 30% Si0 by weight, thesilica particles having an averageparticle size of about 12 to 15 millimicrons and, a specific surfacearea of about 215 mlg. is added to g. 50 percent chlorhydrol and 295 g.water in a blender. The clear sol is then warmed at 90." C. for 15minutes, cooled, and finely powdered Mg (Ol-1) is fed in as dry solid.0.143 equivalents base per equivalent Al is added. The stable productcontains 20.1% SiO 2.67% A1 0 1.0% Cl, and 0.15% MgO, corresponding to amol ratio of Al to surface Si0 of 1:1. The conductivity is 22,300micromhos/cm. at 75 F., and the viscosityis 6.0 cp.

EXAMPLE 6 Example 1 is repeated using a dilute solution of methylaminein water to add 0.165 equivalent base per equivalent Al.

EXAMPLE 7 Example 1 is repeated using a dilute solution ofmonoethanolamine in water to add 0.155 equivalent base per equivalentAl.

EXAMPLE 8 Example 1 is repeated using a solution of sodium hydroxide inwater to add 0.132 equivalent base per equivalent Al.

EXAMPLE 9 Example 5 is repeated using a finely powdered calciumhydroxide slurried in water to add 0. 145 equivalent base per equivalentAl.

EXAMPLE 10 Example is repeated using a dilute solution of lithiumhydroxide to add 0.150 equivalent base per equivalent Al.

EXAMPLE 1 1 Two thousand grams of the silica sol described in Example 3is diluted with 500 g. water to 40% SiO and added to 360 g. 50 percentchlorhydrol and 759 g. water in a blender. The product has a pH of 4.45and a viscosity of 4.0 op.

The product is then allowed to stand at room temperature for 2 weeks,and postadjusted in the blender with solution of potassium hydroxide inwater. 0.145 equivalent base per equivalent Al is added.

The clear, stable sol has 27.6% SiO 2.4% A1 0 and 0.85% C1. The inolratio of Al to surface SiO is 1:1. The viscosity of the sol is 9.0 cp.

EXAMPLE 12 One thousand three hundred and fifty grams of Ludox SMcolloidal silica containing 15% Si0 by weight, the silica particleshaving an average particle size of 7 to 9 millimicrons and a specificsurface area of about 370 m /g, is added to 850 g 25 percent chlorhydrolin a Waring Blendor. The product has a pH of 4.5 and a viscosity of 5.0cps.

The product is then warmed at 90 C. for 10 min., cooled, andpostadjusted in the blender with a dilute ammonium hydroxide solution togive 0.154 equivalent base per equivalent Al.

The clear, very stable sol has a composition of 19.2% SiO 4.6% A1 0 and1.7% C]. The mol ratio of Al to surface SiO is 2:1. Viscosity of the solis 4.0 cp, and specific conductivity is 22,000 micromhos/cm.

EXAMPLE l3 Fifteen hundred grams of Ludox LS colloidal silica describedin Example 5 is added to 130 g 50 percent chlorhydrol and 720 g water ina Waring Blendor. The clear sol is warmed at 60 C. for 2 hours, cooled,and 0.125 equivalent base per equivalent Al is added to the blenderusing dilute ammonium hydroxide solution. The stable product contains19.1% SiO 1.33% A1 0 and 0.5% Cl, corresponding to a mo] ratio of Al tosurface SiO of 1:2.

EXAMPLE l4 cooled. A 17 percent slurry of0.22-lb. MgO in water is addedover one-half hour with good agitation. The amount added is sufficientto give 0.20% MgO in the finished product plus a 10 percent excess toallow for incomplete reaction. Following the MgO addition, the sol isheld at room temperature for one-half hour. Solid particles are filteredfrom the product. The product contains 26.0% SiO 4.0% A1 0 0.20% MgO.

What is claimed is:

1. In a process for preparing a positively charged alumina coated silicasol where an alkaline aquasol of colloidal silica particles is mixedwith an aqueous solution of basic aluminum chloride to yield thepositively charged silica sol, the improvement imparting stability tosaid positively charged silica sol which comprises heating the mixtureat a temperature within the range of 45 C. until the pH of the mixturereaches an essentially constant value and then adding to said mixturefrom 0.032 to 0.223 equivalents of a basic material selected from thegroup consisting of an alkali metal base, an alkaline earth metai base,ammonium hydroxide, a water soluble organic base and mixtures thereofper equivalent of aluminum.

2. An improved process as in claim 1 where said basic aluminum chlorideis Al (OH) ',Cl.

3. An improved process, as in claim 2, where saidcolloidal silicaparticles have an average particle size of from 7 to 30 millimicrons andthe mol ratio of aluminum to surface SiO of said silica particles isfrom 1:2 to 2: l r

4. An improved process, as in claim 1, where said constant pH is 4.2 orbelow.

5. An improved process as in claim 1 where said basic material ismagnesium hydroxide, magnesium ox-- ide, sodium hydroxide, potassiumhydroxide, lithium hydroxide, calcium hydroxide, ammonium hydroxide,methylamine or monoethanolamine.

6. An improved process as in claim 5 where said basic material ismagnesium hydroxide.

7. An improved process as in claim 5 where said basic material is adilute solution of ammonium hydroxide.

8. An improved process as in claim 5 where from 0.09 to 0.17 equivalentsof said basic material is added to the mixture per equivalent ofaluminum.

9. A positively charged silica sol containing an aged mixture ofnegatively charged alumina coated silica sol at a mo] ratio of aluminumto surface SiO of from about 1:2 to about 2:1 which is stabilized withabout 0.032 to about 0.223 equivalents of an alkali metal base, analkaline earth metal base, ammonia, a water soluble organic base ormixture thereof per equivalent of aluminum in the aged mixture.

1. In a process for preparing a positively charged alumina coated silicasol where an alkaline aquasol of colloidal silica particles is mixedwith an aqueous solution of basic aluminum chloride to yield thepositively charged silica sol, the improvement imparting stability tosaid positively charged silica sol which comprises heating the mixtureat a temperature within the range of 45* - 90* C. until the pH of themixture reaches an essentially constant value and then adding to saidmixture from 0.032 to 0.223 equivalents of a basic material selectedfrom the group consisting of an alkali metal base, an alkaline earthmetal base, ammonium hydroxide, a water soluble organic base andmixtures thereof per equivalent of aluminum.
 2. An improved process asin claim 1 where said basic aluminum chloride is Al2(OH)5Cl.
 3. Animproved process, as in claim 2, where said colloidal silica particleshave an average particle size of from 7 to 30 millimicrons and the molratio of aluminum to surface SiO2 of said silica particles is from 1:2to 2:1.
 4. An improved process, as in claim 1, where said constant pH is4.2 or below.
 5. An improved process as in claim 1 where said basicmaterial is magnesium hydroxide, magnesium oxide, sodium hydroxide,potassium hydroxide, lithium hydroxide, calcium hydroxide, ammoniumhydroxide, methylamine or monoethanolamine.
 6. An improved process as inclaim 5 where said basic material is magnesium hydroxide.
 7. An improvedprocess as in claim 5 where said basic material is a dilute solution ofammonium hydroxide.
 8. An improved process as in claim 5 where from 0.09to 0.17 equivalents of said basic material is added to the mixture perequivalent of aluminum.